DIATOMACEiE. 



[ 202 ] 



DIATOMACEiE. 



the cell-contents of Closterium, namely en- 

 cjsted globules (PI. 6. fig. 10) resembling the 

 resting -spores of Volvox and the filamentous 

 Confervae (CEdogonium), and he considers 

 that such bodies produced in S. bifrons may 

 probably reproduce S. Microcora. In some 

 of Mr. Thwaites's figures of conjugating 

 Diatomacese (PI. 6. fig. 6), there are appear- 

 ances which would lead to the idea that spores 

 were occasionally produced in this process. 



The principal attraction of the Diatoma- 

 ceae to microscopists lies, however, at present 

 in the structure of the siliceous coats, and we 

 must devote some considerable space to that 

 part of the subject. 



Some remarks upon the method of render- 

 ing the markings visible have been made in 

 the Introduction, p. xxv {Illumination); 

 and upon the cause of their becoming visible 

 under proper illumination in the article 

 Angular Aperture. The grounds for 

 the belief that most of the markings are 

 depressions, have also been mentioned (In- 

 troduction p. xxxiii, /.). Difi'erent views 

 of the nature of the markings from those 

 entertained by us have been proposed by 

 other authors ; but these appear based upon 

 no kind of evidence whatever, and may be 

 regarded as mere statements without attempt 

 at proof. The last we have met with is that 

 of Schacht, who compares them to the striae 

 upon the liber-fibres of Vinca; it would be 

 difficult to find a more hasty generalization. 

 We shall not dwell upon these debates, but 

 proceed to some further instructions for 

 observing the objects. 



Preparation of the valves, to render them 

 as distinct as possible, is essential. This may 

 be effected in two ways : — 1. By incinerating 

 them upon a very thin plate of mica over 

 the flame of a spirit-lamp. This is the 

 quickest method, but it has the disadvan- 

 tages of the valves often becoming semi- 

 fused or agglutinated to each other by the 

 eff"ects of the heat in the presence of the 

 alkaline salts contained in all organic matters, 

 especially those which are of marine origin. 

 2. Boiling with strong nitric acid. This is 

 the best method. The water containing the 

 Diatomaceae is allowed to settle for twenty- 

 four hours, the supernatant liquid poured 

 off and the deposit dried in a porcelain dish. 

 Strong nitric acid is then added, the whole 

 mixed A^ith a feather or glass brush, and 

 poured into a flask or test-tube and boiled 

 for some time; a portion being removed 

 occasionally with a dip-tube to determine 

 when the valves are perfectly clean. When 



this is the case, distilled water is added to 

 the mixture, and the whole allowed to settle. 

 The supernatant liquid is then carefully 

 decanted, more water added, and the mixture 

 again allowed to settle, poured off", and these 

 operations repeated until a drop of the liquid 

 containing the valves, when evaporated on a 

 slide, leaves no film (of calcareous salts) at 

 the margins of the drop. This is a some- 

 what tedious process, but it is essential that 

 it should be thoroughly carried out. If the 

 valves be not thoroughly w ashed, the film of 

 nitrate of lime remaining upon the shde will 

 absorb water fi'om the atmosphere, and the 

 whole will be spoiled. 



The appearance of the valves thus prepared 

 will vary according to their structure, and the 

 manner in which they are examined. In 

 some cases the valves appear colom*less, and 

 the markings perfectly distinct with the 

 ordinary direct light of the mirror, provided 

 the power be sufficient (PI. 13. fig. 2; PI. 18. 

 figs. 32, 43, 45). In others (PL 11), the 

 valves appear coloured when viewed by the 

 ordinary light. But when the mirror is 

 brought to one side, and the light is thus 

 thrown upon the object obliquely, one or 

 two sets of fine parallel lines are seen traver- 

 sing the valves (PL 1. figs. 17, 18; PL 11. 

 figs. 10, 12, 15, &c.). And when an object- 

 glass of considerable aperture is used, with 

 the condenser and central stop exactly cen- 

 trical (Introduction, p. xvi), the lines 

 are replaced entirelv or in part by a series of 

 dots (PL 1. fig. 16;' PL 11. figs. 39,40,&c.); 

 these, under a high eye-piece, have di- 

 stinctly angular forms, sometimes appear- 

 ing regularly hexagonal (PL 11. figs. 41,48). 

 If the condenser and stop be not exactly 

 centrical, or the surface of the valve be not 

 flat, the true form of the dots will be replaced 

 by some other ; thus hexagonal dots may be 

 made to appear triangular, quadrangular, &c., 

 and those dots which cannot be conceived to 

 be really hexagonal (PL 1 1. fig. 39) may be 

 made to appear so. 



Those who do not possess object-glasses of 

 considerable aperture, may render evident 

 the lines or dots upon many of the common 

 and former^ considered difficult valves, by 

 using a central stop in both the condenser 

 and the object-glass (Introduction, p. xvi). 



There can be but little doubt that the 

 valves of all the Diatomaceae are furnished 

 with markings, although in some of them 

 they have not yet been detected. In the 

 most difficultly resolvable of those at present 

 known, lines onlv can be rendered evident. 



